--> --> Fractures and Vein-Fill Evolution of the Middle Devonian Marcellus Shale

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Fractures and Vein-Fill Evolution of the Middle Devonian Marcellus Shale


A vertical core containing the entire thickness of the Middle Devonian Marcellus Shale from the MIP-3H well in West Virginia was used to investigate the morphological evolution of natural fractures and associated vein-fill. The evolutionary process of self-induced fracture formation is explained by the relationship between the fracture and its vein cement morphology along with the number of vein generations. The Marcellus Shale in the MIP-3H core is within the dry gas window at 7,447 to 7,557 ft (2,270 to 2,303 m) depth. Gamma-ray shows a general decreasing upwards trend in total organic carbon (TOC) from 15 to ~2%. A medical computed tomography (CT) scanner was used to create 2D and 3D visualizations of fractures. Formation micro imaging (FMI) log analysis was also used. Petrographic microscopy on nine large format thin sections containing healed fractures in the host shale were used to describe vein cement morphology and determine the number of vein generations. Cathodoluminescence (CL) microscopy will be used to determine fracturing, zoning or recrystallization induced by compositional changes in the calcite veins. A variety of healed fracture morphologies were documented: bedding-parallel, sub-horizontal, vertical (continuous and discontinuous), sub-vertical, and deformation. The lower section of the core contained numerous and assorted fracture morphologies and the cross-cutting relationships were more complex relative to the rest of the core. Petrographic observation of the thin sections showed crack-seal structures associated with crystal growth. Continuous bitumen layers (<1 to 5 mm) were observed predominantly lining one side of veins. Calcite vein thickness ranged from <1 to 3.5 mm. Cement morphologies documented were bladed fibrous, acicular fibrous, coarse-grained euhedral to subhedral, and fine-grained. The number of observable vein generations varied from one to five episodes of mineralization. In thin section, the smallest number of mineralization episodes and morphology types occurred in bedding-parallel fractures, which contained one generation of bladed fibrous calcite fill ~0.75 mm thick. Vertical discontinuous fractures contained the most with five mineralization episodes; three generations of coarse-grained and two generations of acicular fibrous. Individual fracture morphologies have their own unique combinations of vein morphology, number of mineralization episodes, and bitumen thickness, which could relate to the evolution of self-induced fractures during maturation of the Marcellus.